The present invention relates to aircraft control systems and more particularly to automatic flight control systems which allow normal manual maneuvering during automatic attitude or navigational course control.
A highly desirable function of an aircraft control system is to provide automatic maintenance of attitude by control of aircraft pitch, roll and yaw axes or automatic maintenance of a navigational course or flight path through programmed control of those axes. During operation with automatic control systems, however, it is also desirable to enable the pilot to provide input commands through manual operation of the control stick while still allowing the aircraft to return to its automatic control and previous attitude or course following manual maneuvering. While the above dual control is desirable, prior art systems have encountered great difficulty in providing automatic systems which enable natural aircraft response to manual inputs during operation in the automatic mode.
In one prior art system, the automatic flight control system (AFCS) uses full time attitude command augmentation to implement the control system and allow manual override through pilot input. In these systems, the memorized attitude reference or navigational command in a selected axis is provided to control the system at the same time that the pilot input is being implemented through the control stick. As a result, a stick force or displacement is required which is proportional to attitude thereby resulting in a requirement that large forces or displacements be utilized to overcome the predetermined attitude commands. The large forces or displacements which are required cause an unnatural response by the aircraft to control stick inputs.
In still other systems, an automatic flight control with rate command augmentation and attitude hold functions is utilized to allow manual control of the aircraft by natural stick movement during automatic flight. In such systems, however, the AFCS maintains a current attitude reference through control of the selected channels when the aircraft control stick is in a neutral position and not being maneuvered by the pilot. When the control stick is moved, the aircraft responds in a natural way with a rate of change which is constant and proportional to stick displacement. However, upon release of the aircraft control stick following the manual fly-through maneuvering, the AFCS maintains the attitude reference last achieved by the pilot rather than returning to the originally commanded attitude reference or navigational course. This prevents a pilot from manually maneuvering the aircraft while still allowing the AFCS to return the aircraft to the previous attitude reference or navigational course following that manual input. Accordingly, if a pilot becomes disoriented or the control stick is accidentally moved, the aircraft cannot regain the original attitude reference or navigational course without pilot assistance.
In each of the above cases, there are deficiencies and limitations on aircraft operation. Specifically, the first system requires unnatural stick displacement resulting in feedback to the pilot which is not similar to aircraft response during normal manual control. Such feedback may be confusing and result in improper operation of the aircraft during critical flight periods of automatic control or reversion to manual control. Likewise, in the second example, the system is not capable of returning to a predetermined attitude reference or flight path which may be desirable under various flight conditions. Thus, although the aircraft responds naturally to aircraft control stick movements without an unnatural feel, the cost of providing that natural response is the loss of a return to a predetermined attitude reference or navigational course. The systems therefore fail to provide needed control during automatic flight conditions to allow more safe and effective operation of the aircraft in various environments.
The present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide a system and technique which allows normal manual control during automatic flight and return to a predetermined attitude reference or navigational course following manual maneuvering.